Hello,
Since from 18th of December 2017, one day after, let's encrypt certificate renew.
Ios mail client over iphone 6, 6plus, 7, 7plus and 5se
says the following ...Let me show you 3 images.
The domain is laviruta.com and the server domain is sofihacloud.com.ar
At first I used another email client, spark for ios, and works fine,
but some of my clients want use the ios mail client because is more familiar for them.

Yes,
But I have some several troubles with the links on certificates folders, I had applied this configuration to all my 6 ISPConfig Servers

Remember, my distro is CentOS 6.9 in this Server and onothers Servers is CentOS 7.4

I had wrote you about this in some post before.
========================================
Let me remember that..
========================================I have solved, doing the following
[Using The Same Let's Encrypt SSL Certs For Other Major Services]
/etc/postfix/main.cf
###smtpd_tls_cert_file = /etc/postfix/smtpd.cert
###smtpd_tls_key_file = /etc/postfix/smtpd.key
smtpd_tls_cert_file = /etc/letsencrypt/live/sofiha-isp.com.ar/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/sofiha-isp.com.ar/privkey.pem

I don't use any Apple deivices, so I can't tell you what's the exact problem. But there are many reports out there that apple mail has problems with LE certs. Waht you can try is to check which LE chain certs are in the fullchain cert and maybe add some affitional ones: https://letsencrypt.org/certificates/
Chain certs in postfix and dovecot can also be added in a separate file and loaded in postfix with: